(1) Field of the Invention
The present invention relates to a power supply voltage detecting apparatus for detecting a drop in a power supply voltage below a specified level.
(2) Description of the Prior Art
Dry batteries are generally used as a power supply in a portable electric equipment unit such as a portable audio cassette tape player. When the energy of a dry battery is dissipated to an extent that the output voltage thereof drops to a certain level, there arises problems such as a degradation in a reproduced sound quality due to an unstable performance of a motor for running a tape. Hence, in this equipment unit means are provided for stopping a playback operation when a drop in the output voltage of a dry battery is detected.
FIG. 1 shows an essential part of such a portable electric equipment unit as mentioned above.
Referring to FIG. 1, a controlling part 1 controls the operation of this electric equipment unit; an operation display part 2 allows a user to operate this electric equipment unit; and a mechanism part 3 includes a motor and other driving parts of this electric equipment unit.
The power supply voltage output from a dry battery 4 is applied to the controlling part 1, the operation display part 2 and the mechanism part 3. A voltage detecting circuit 5 detects a drop in the output voltage of the dry battery 4 below a specified level, a detection signal DT being output therefrom to the controlling part.
Therefore, the controlling part 1 executes specified operations in accordance with an operational signal output from the operation display part 2 and stops an operation currently being performed when the detection signal DT output from the voltage detecting circuit 5 switches to a signal indicating that a low voltage has been detected.
FIG. 2 shows an example of the voltage detecting circuit 5.
Referring to FIG. 2, a reference voltage signal Vref output from a reference voltage generating circuit 7 is fed to the negative terminal of a difference amplifier 6; a fraction of a power supply voltage Vdd obtained by means of a voltage divider comprising resistors 8 and 9 is fed to the positive terminal of the difference amplifier 6.
Therefore, when the power supply voltage Vdd is at a sufficiently large level and the fraction of the power supply voltage obtained by means of the voltage divider comprising the resistors 8 and 9 is larger than the reference voltage Vref, the detection signal DT output from the difference amplifier 6 is at a logical level H.
When the energy of the dry battery 4 is dissipated and the level of the power supply voltage Vdd drops to such an extent that the fraction of the power supply voltage obtained by means of the voltage divider comprising the resistors 8 and 9 is smaller than the reference voltage Vref, the detection signal output from the difference amplifier 6 is at a low level L.
Electric equipment driven by dry batteries is configured such that a main circuit thereof is constructed of CMOS-type semiconductor devices so as to ensure a long life for the dry batteries. Therefore, the voltage detecting circuit 5 in such equipment is configured as shown in FIG. 3. In the figure, those parts that are identical to or correspond to the parts of FIG. 2 are designated by the same reference numerals as in FIG. 2.
Referring to FIG. 3, p-channel MOS transistors 11, 12 and n-channel MOS transistors 13, 14 form a CMOS-type difference amplifier. A constant-current source 15 is provided so as to ensure a stable operation of this difference amplifier.
The reference voltage Vref output from the reference voltage generating circuit 7 is applied to thea gate of the n-channel MOS transistor 13. The fraction of the power supply voltage obtained by means of the voltage divider comprising the resistors 8 and 9 is applied to the gate of the n-channel MOS transistor 14. A signal present at a junction between the p-channel MOS transistor 12 and the n-channel MOS transistor 14 is fed to the input of an output signal generating circuit 16, as an output signal of this difference amplifier circuit.
The output signal generating circuit 16 converts the output signal of the difference amplifier circuit to a logical signal having a specified logical level. The output signal from the output signal generating circuit 16 is then supplied to a circuit in the next stage, as the detection signal DT.
However, the conventional apparatus described above has the following disadvantages.
For example, there is now available, as a result of power dissipation being reduced, a portable audio cassette tape player operable with one size AA battery. In such an equipment unit operable with one size AA battery, the voltage detecting circuit 5 described above is required to be able to detect a voltage in the neighborhood of 0.9 volts.
Since, in the circuit shown in FIG. 3, it is necessary to operate the p-channel MOS transistors 11, 12 and n-channel MOS transistors 13, 14 in a saturation range, the power supply voltage Vdd with which this difference amplifier is operable, i.e. (Vdd-Vss), where Vss is a ground level, is in the range expressed by the following inequality (1). EQU (Vdd-Vss).gtoreq.Vth11+Vref-Vth13+Vd7 (1)
where Vth11 indicates a threshold voltage of the p-channel MOS transistor 11; Vth13 indicates a threshold voltage of the n-channel MOS transistor 13; and Vd7 is a voltage drop across the constant-current source 7.
Thus, the voltage range over which the difference amplifier shown in FIG. 3 is operable is affected by the above four factors, namely, the threshold voltage of the p-channel MOS transistor 11, the threshold voltage of the n-channel MOS transistor 13, the reference voltage and the voltage drop across the constant-current source 7.
Since, normally, variations in these factors are created during a process of fabricating integrated circuits constituting a difference amplifier, some of the fabricated integrated circuits may cause the voltage range, over which range the difference amplifier is operable, to be extended beyond 0.9 volts. That as many as four factors affect the voltage range increases the chances that a defective circuit will be produced, thus resulting in an unsatisfactory production yield and very expensive production costs.